1. Technical Field
The present invention relates to a signal processing device and to a liquid droplet ejection device, and in particular to a signal processing device and liquid droplet ejection device for processing a sensor signal output by a temperature sensor or a humidity sensor.
2. Related Art
Generally, as a temperature detection sensor, thermistors are known whose resistance value changes according to temperature (thermistic sensors). A specific example of the relationship between the resistance value and the temperature of such thermistors is shown in FIG. 8. As shown in FIG. 8, the resistance value of the thermistor changes in room temperature environments by about 50% to 150%, from a central value of 10 kΩ. An example of a temperature sensor circuit for generating a voltage corresponding to the resistance value of the thermistor is shown in FIG. 9. In this temperature sensor circuit, a reference electrical potential Vcc is divided by the resistance of the thermistor and a known resistor, so as to generate a voltage dependent on the resistance value of the thermistor (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-255213).
Generally, as a humidity detection sensor, humidity sensors are known that employ elements whose resistance value changes according to humidity. A specific example of the relationship between the resistance value and the humidity of such humidity sensors is shown in FIG. 10. As shown in FIG. 10, there is a greater amount of change in the resistance value of the humidity sensor compared to that in the thermistor, with a change of 3 to 4 orders of magnitude. An example of a humidity sensor circuit for generating a voltage that corresponds to the resistance value of a humidity sensor is shown in FIG. 11. Since the amount of change in the resistance value in this humidity sensor circuit is large, often logarithmic compression is performed by employing a diode that utilizes the characteristics of a semiconductor PN junction.
FIG. 2 shows an example of an inkjet head provided with one each of a thermistor and a humidity sensor. In the circuit for generating a voltage corresponding to the resistance value of the sensor in this inkjet head is now considered for a case in which the humidity sensor circuit example shown in FIG. 9, and the humidity sensor circuit example shown in FIG. 11, are applied.
This inkjet head is formed with two circuits that are electrically the same as each other. These two circuits only differ in whether the type of sensor mounted is a thermistor, or a humidity sensor. The interface are taken to be the same, irrespective of the type of sensor. Consequently, it is necessary to detect both resistance values of a thermistor and resistance values of a humidity sensor, respectively, using the same circuit. There is a memory mounted to the inkjet head. The information indicating whether a thermistor or a humidity sensor is mounted is stored in this memory.
Were the humidity sensor circuit shown in FIG. 9 to be applied as a temperature sensor circuit, the humidity sensor circuit would be required to apply an alternating voltage of a specific amplitude (for example, 1 Vpp) at 1 kHz to the humidity sensor. Consequently, the reference electrical potential Vcc would need to be transformed into an alternating current power source. However, a bias voltage applied to the sensor changes depends on the ratio of the resistor R1 to the sensor resistance, and so an alternating voltage of a specific amplitude cannot be applied. Furthermore, the resistance value of a humidity sensor changes by 3 to 4 orders of magnitude as described above, so the dynamic range of the voltage output must be of this order or greater. Furthermore, high speed responsiveness is required of the circuit itself in order to correspond to an alternating bias of 1 kHz. However, it is generally difficult to achieve both a high dynamic range (low noise) and high speed properties at the same time. Therefore these problems arise when a temperature sensor circuit is applied as a humidity sensor circuit.
On the other hand, were the humidity sensor circuit shown in FIG. 11 to be applied as a temperature sensor circuit, logarithmic transformation would be performed on the resistance value of the temperature sensor. Therefore, in order to increase the resolution of temperature detection, the dynamic range of the voltage output would need to be increased. Furthermore, in order to correspond to an alternating bias of 1 kHz, a high speed response is desired, similarly to when detecting temperature. Consequently, it is similarly difficult to achieve both a high dynamic range (low noise) and high speed at the same time. Therefore problems arise when a humidity sensor circuit is applied as a temperature sensor circuit.